<p>The using namespace directive imports names from the given
      namespace and when used in a header file influences the global
      namespace of all the files that directly or indirectly include
      this header file.</p>

<p>It is imaginable to use the using namespace directive in a
      limited scope in a header file (for example in a template or
      inline function definition), but for the sake of consistency
      this is also discouraged.</p>
